Electrode Sheet and Method for Manufacturing the Same

This application claims priority from Japanese Patent Application No. 2024-094650 filed on Jun. 11, 2024. The entire content of the priority application is incorporated herein by reference.

The technology disclosed herein relates to an electrode sheet and a method of manufacturing the same.

Japanese Unexamined Patent Application Publication No. 2021-504877 (JP 2021-504877 A) describes an electrode sheet for a battery cell and a method for manufacturing the same. The electrode sheet includes active material particles and a binder. The method of manufacturing the electrode sheet includes a step of producing an electrode mixture by mixing the active material particles with the binder and a step of manufacturing the electrode sheet using the electrode mixture.

Adding a conduction aid to an electrode sheet such as the one described above allows for improvement in the conductivity of the electrode sheet. The conduction aid is, for example, carbon black or carbon nanotube, and forms conductive pathways extending between active material particles. However, when the active material particles are mixed together with the conduction aid and a binder, the conduction aid may be incorporated into the binder. In this case, the amount of conduction aid extending between the active material particles is reduced, thereby failing to improve the conductivity of the electrode sheet sufficiently.

To avoid the above problem, it is envisioned to reduce the amount of binder to be mixed. However, the reduction in the amount of binder to be mixed may lead to a decrease in the tensile strength of the electrode sheet. Thus, to improve the tensile strength of the electrode sheet, a fibrillatable binder such as polytetrafluoroethylene (PTFE) may be used. However, when the binder is fibrillated, the conductive aid is more likely to be entangled in the binder. As a result, a larger amount of the conduction aid may be incorporated into the binder, further reducing the amount of conduction aid extending between the active material particles.

In view of the above, the disclosure herein provides a technology for improving the conductivity of an electrode sheet while maintaining its tensile strength.

The technology disclosed herein is embodied as a method of manufacturing an electrode sheet. In a first aspect, the method may comprises producing first coated active material particle s by mixing first active material particles with at least a binder; producing second coated active material particles by mixing second active material particles with at least a conduction aid; and producing an electrode mixture by mixing the first coated active material particles with the second coated active material particles; and forming the electrode mixture into a sheet shape.

The manufacturing method described above comprises a step of producing the first coated active material particles and a step of producing second coated active material particles. The electrode mixture is produced by mixing the first coated active material particles with the second coated active material particles. In the step of producing the first coated active material particles, the first active material particles and the binder can be mixed together without giving consideration to the effect on the conduction aid. Further, in the step of producing the second coated active material particles, the conduction aid can be adhered to the second active material particles without being affected by the binder. This prevents the conduction aid from being incorporated into the binder when the first coated active material particles and the second coated active material particles are mixed together in the step of producing the electrode mixtures. Thus, a relatively large amount of conduction aid can be used to form conductive pathways between the active material particles without a reduction in the amount of binder to be mixed, allowing for manufacturing of an electrode sheet excellent in both tensile strength and conductivity.

In a second aspect according to the first aspect, producing the first coated active material particles may comprise fibrillating the binder by applying a shear force to at least the binder. This configuration allows for a further improvement of the tensile strength of the electrode sheet.

In a third aspect according to the first or second aspect, the shear force applied to at least the binder in fibrillating the binder may be larger than a shear force applied to the second active material particles and the conduction aid in producing the second coated active material particles. This configuration improves binding between the first active material particles and the binder in the first coated active material particles.

In a fourth aspect according to any of the first to third aspects, the conduction aid may comprise at least one component selected from the group consisting of carbon nanotube and acetylene black. The carbon nanotube has a tube shape and the acetylene black has a chain-like structure. Using a conduction aid with such a shape or structure makes it easier for the conduction aid to intertwine with each other and thus facilitates the formation of conductive pathways. On the other hand, a conductive aid having a tube shape or chain structure is easily incorporated into the binder. However, in the present technology, since the conduction aid adheres to the second active material particles in advance in the step of producing the second coated active material particles, the conduction aid is effectively suppressed from being incorporated into the binder.

In a fifth aspect according to any of the first to fourth aspects, the electrode sheet may be a freestanding electrode sheet. This configuration allows for an increase in the energy density of an electrode. A freestanding electrode sheet herein means an electrode sheet that supports itself without the need for a support (e.g., a current collector).

The technology disclosed herein is also embodied as an electrode sheet. The electrode sheet can be manufactured by the manufacturing method described above. For example, in a sixth aspect, the electrode sheet may comprise active material particles; a binder that binds the active material particles together in a sheet shape; and a conduction aid that forms a conductive path extending between the active material particles in the sheet shape. The active material particles may comprise first active material particles and second active material particles. At least a part of a surface of each first active material particle may be coated by the binder, and at least a part of a surface of each second active material particle may be coated by the conduction aid. As described above, the electrode sheet manufactured by the present technology is excellent in both tensile strength and electrical conductivity.

In a seventh aspect according to the sixth aspect, the binder may be a fibrillated resin. This configuration allows for further improvement in the tensile strength of the electrode sheet.

In an eighth aspect according to the sixth or seventh aspect, the conduction aid may comprise at least one component selected from the group consisting of carbon nanotube and acetylene black. In this configuration, the conduction aid has a tube shape or chain structure, which improves the conductivity of the electrode sheet.

In a ninth aspect according to any of the sixth to eighth aspects, the electrode sheet may be a freestanding electrode sheet. This configuration allows for an increase in the energy density of an electrode.

Representative, non-limiting examples of the present disclosure will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing aspects of the present teachings and is not intended to limit the scope of the present disclosure. Furthermore, each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide improved electrode sheets, as well as methods for manufacturing the same.

Moreover, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the present disclosure in the broadest sense, and are instead taught merely to particularly describe representative examples of the present disclosure. Furthermore, various features of the above-described and below-described representative examples, as well as the various independent and dependent claims, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.

All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter.

With reference to the drawings, an electrode sheetaccording to an embodiment will be described. The electrode sheetaccording to the embodiment is used in an electrode body. The electrode bodyis used, for example, as a positive electrode of a lithium-ion secondary battery.

As shown in, the electrode bodycomprises the electrode sheetand a current collector. The current collectoris a conductive sheet. The current collectoris, for example, an aluminum foil or a copper foil. The thickness of the current collectoris, for example, 5 μm or more and 50 μm or less. The electrode sheetis located on the current collector. The electrode sheetis a freestanding electrode sheet. A freestanding electrode sheet herein means an electrode sheet that supports itself without requiring a support such as the current collector. Thus, the electrode bodydoes not necessarily need to comprise the current collector. That is, in another embodiment, the electrode sheetmay constitute the electrode bodyby itself. The thickness of the electrode sheetis, for example, 10 μm or more and 500 μm or less.

As shown in, the electrode sheetcomprises active material particles,, a binder, and a conduction aid. The active material particles,comprise first active material particlesand second active material particles. At least a part of the surface of each first active material particleis coated by the binder. At least a part of the surface of each second active material particleis coated by the conduction aid.

The active material particles,each are positive-electrode active material particles since the electrode sheetis used as a positive electrode of a lithium-ion secondary battery in this embodiment as described above. Examples of the active material particles,include, for example, lithium composite oxides. Examples of the lithium composite oxides include, for example, lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel manganese composite oxides (e.g., LiNiMnO), lithium nickel manganese cobalt composite oxides (e.g., LiNiMnCoO), and the like. The active material particles,may be constituted of a single material or multiple materials. The compound used as the first active material particlesmay be the same as or different from the compound used as the second active material particles.

The bindercan bind between the active material particlesand. Examples of the binderinclude, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), acrylic resin, ultra-high molecular weight polyethylene, and the like. The bindermay be constituted of a single material or multiple materials. The binderin this embodiment is a fibrillatable resin, and the resin is fibrillated in the electrode sheet. A fibrillatable resin herein means a resin that can be fibrillated by a shear force being applied thereto. Examples of the fibrillatable resin include, for example, celluloses, acrylic resins, ultra-high molecular weight polyethylene, and PTFEs.

The conduction aidcan form conductive pathways extending between the active material particles,in the electrode sheet. Examples of the conduction aidinclude, for example, carbon nanotubes, carbon black (e.g., acetylene black, furnace black, ketjen black, etc.), cokes, and graphite carbon materials. The conduction aidmay be constituted of a single material or multiple materials.

Referring now to, a manufacturing method of the electrode sheetwill be described. This manufacturing method allows for manufacturing of the electrode sheetwithout using any solvents. That is, the manufacturing method is a so-called dry process.

As shown in, the manufacturing method comprises a step of producing first coated active material particles by mixing the first active material particleswith the binder(S). In this step, a mixeris used, for example, as shown in. The mixermixes the first active material particlesand the binderput into a containerby rotating a blade. Thereby, the first coated active material particles in which the binderis adhering to the first active material particlesis produced. That is, in the first coated active material particles, at least a part of the surface of each first active material particleis coated by the binder. In this step, in addition to the first active material particlesand the binder, other necessary materials may be mixed together. However, the conduction aidis not mixed in this step. The absence of the conduction aidallows for the mixing of the first active material particlesand the binderwithout giving consideration to the effect on the conduction aid.

In this embodiment, the mixerincreases the rotation speed of the bladein stages such that the first active material particlesare mixed with the binderat two different rotation speeds. However, the mixerdoes not necessarily need to increase the rotation speed of the bladein two stages. In another embodiment, the rotation speed of the blademay be constant or increased in three or more stages. In S, the mixerneed not necessarily be used. In other embodiments, another mixer such as a blender, a mill, or the like may be used in place of the mixer.

As shown in, the manufacturing method further comprises a step of fibrillating the binderby applying a shear force to the first coated active material particles (S). In this step, a kneaderis used, for example, as shown in. The kneaderapplies a shear force to the first coated active material particles between a bladeand a wallof a containerby rotating the blade. As described above, since the binderin this embodiment is a fibrillatable resin, the binderis fibrillated by the shear force being applied to the binderwhich constitutes the first coated active material particles. This allows for further improvement in the tensile strength of the electrode sheet. In S, the kneaderneed not necessarily be used. In other embodiments, another mixer such as a blender, a mill, or the like may be used in place of the kneader. The step Smay be performed with the containerof the kneader heated at a predetermined temperature, although this need not necessarily be the case.

As shown in, the manufacturing method further comprises a step of producing second coated active material particles by mixing the second active material particleswith the conduction aid(S). In this step, a mixer is used, for example, as shown in. The mixer mixes the second active material particlesand the conduction aidput into a container by rotating a blade. Thereby, the second coated active material particles in which the conduction aidis adhering to the second active material particlesis produced. That is, in the second coated active material particles, at least a part of the surface of each second active material particleis coated by the conduction aid. In this step, in addition to the second active material particlesand the conduction aid, other necessary materials may be mixed together. In this embodiment, PVdF is mixed together, although this is merely an example. The PVdF is an additive for bonding between the second active material particlesand the conduction aidduring the mixing of the second active material particlesand the conduction aid. That is, the PVdF used in this step is not intended to bind between the active material particlesandin the electrode sheet, as the binderused in Sdoes. Thus, the amount of PVdF to be mixed is relatively small, and the conduction aidis not incorporated into the PVdF. Materials other than the PVdF may be used as such additives, but fibrillatable resins such as PTFE, for example, should not be used.

As shown in, the manufacturing method further comprises a step of producing an electrode mixture by mixing the first coated active material particles with the second coated active material particles (S). In this step, a mixer is used, for example, as shown in. By rotating a blade of the mixer, the first coated active material particles and the second coated active material particles put into a container are mixed together. Thereby, the electrode mixture is produced. Producing the electrode mixture in this manner suppresses the conduction aidfrom being incorporated into the binder, for example, compared to when the electrode mixture is produced by mixing the active material particles,, the binder, and the conduction aidtogether all at once.

As shown in, the manufacturing method further comprises a step of forming the electrode mixture into a sheet shape (S). In this step, a press deviceis used, for example, as shown in. The press deviceis equipped with a pair of rollersand is configured to roll the electrode mixture while it is passing between the pair of rollers. The electrode mixture is thus formed into a sheet shape by being rolled by the pair of rollers. Thereby, the electrode sheetis manufactured. As described above, the resulting electrode sheetis a freestanding electrode sheet. The step Smay be performed with the pair of rollersheated at a predetermined temperature, although this need not necessarily be the case.

According to the manufacturing method described above, a relatively large amount of conduction aidcan extend and form conductive pathways between the active material particlesandwithout a reduction in the amount of binderto be mixed, so that the electrode sheetwith both excellent tensile strength and conductivity can be manufactured.

The average particle diameter of the active material particles,used to manufacture the electrode sheetis, for example, 1 μm or more and 25 μm or less, or 1.5 μm or more and 20 μm or less. The average particle diameter herein means a particle diameter at 50% integration (D50) in a volume-based particle size distribution measured by the laser diffraction/scattering method. The size relationship between the average particle diameter of the first active material particlesand the average particle diameter of the second active material particlesis not particularly limited. Further, a single kind of positive-electrode active material particles having any average particle diameter may be used as the active material particlesand, or two or more kinds of positive-electrode active material particles having different average particle diameters may be used as the active material particlesand.

The content of the first active material particlesand the second active material particlesin the electrode mixture is, for example, 90 weight % or more and 99 weight % or less, or 95 weight % or more and 98.5 weight % or less. The weight ratio of the content of the first active material particlesto the content of the second active material particlesin the electrode mixture is, for example, 3:7, 4:6, 5:5, 6:4, or 7:3.

The content of the binderin the electrode mixture is, for example, 0.5 weight % or more and 5 weight % or less, or 1 weight % or more and 3 weight % or less. The content of the conduction aidin the electrode mixture is, for example, 0.25 weight % or more and 3 weight % or less, or 0.5 weight % or more and 2 weight % or less. The content of the additive in the electrode mixture is, for example, 0.1 weight % or more and 2 weight % or less, or 0.25 weight % or more and 1 weight % or less.

In the manufacturing method described above, the shear force applied to the binderin the step of fibrillating the binder(S) is greater than the shear force applied to the second active material particlesand the conduction aidin the step of producing the second coated active material particles (S). This configuration increases the binding between the first active material particlesand the binderin the first coated active material particles. Thus, the tensile strength of the electrode sheetcan be improved.

In one example, the conduction aidused in the above manufacturing method comprises at least one component selected from the group consisting of carbon nanotube and acetylene black. The carbon nanotube has a tube shape and the acetylene black has a chain-like structure. Using the conduction aidhaving such a shape or structure makes it easier for the conduction aidto intertwine with each other and thus facilitates the formation of conductive pathways. On the other hand, a conductive aidhaving a tube shape or chain structure is easily incorporated into the binder. However, in the present technology, since the conduction aidadheres to the second active material particlesin advance in the step of producing the second coated active material particle (S), the conduction aidis effectively suppressed from being incorporated into the binder.

Examples related to the present technology will be described below, however, it is not intended to limit the technology to such examples.

As each of the active material particlesand, a mixture of LiCoNiMnOmonocrystals (hereinafter referred to as NCM monocrystals, average particle diameter: 3 μm) and LiCoNiMnOpolycrystals (hereinafter referred to as NCM polycrystals, average particle diameter: 10 μm) was used. As the binder, powder of polytetrafluoroethylene (PTFE, Chemours Company) was used. As the conduction aid, powder of carbon nanotubes (CNT, LG Chem Ltd.) was used. As an additive, powder of polyvinylidene fluoride (PVdF, Arkema S.A.) was used. The weight ratio of NCM monocrystals/NCM polycrystals/PTFE/CNT/PVdF was 48.7/48.7/1.4/0.75/0.5. That is, each of the active material particlesandwas a mixture of NCM monocrystals and NCM polycrystals, and the weight ratio of the first active material particlesto the second active material particleswas 5:5. Hereafter, the mixture of NCM monocrystals and NCM polycrystals used as the first active material particlesis referred to as NCM-1, and the mixture of NCM monocrystals and NCM polycrystals used as the second active material particlesis referred to as NCM-2.

As shown in, the NCM-1 and the PTFE were first put into a mixer (MP5B, Nippon Coke Co., Ltd.) and mixed at 300 rpm for 180 seconds, and then mixed at 5000 rpm for 500 seconds. The first coated active material particles were thereby produced. The first coated active material particles were put into a kneader (DSI-5, Nihon Spindle Manufacturing Co.) and kneaded at 10 rpm for 180 seconds at 100° C. This gave a relatively large shear force to the first coated active material particles, fibrillating the PTFE.

The NCM-2, the CNT, and the PVdF were put into a mixer (MP5B, Nippon Coke Co., Ltd.) and mixed at 10000 rpm for 10 minutes. The second coated active material particles were thereby produced.

The first coated active material particles and the second coated active material particles were put into a mixer (MP5B, Nippon Coke Co., Ltd.) and mixed at 300 rpm for 1 minute. The electrode mixture was thereby produced.

The electrode sheetwas manufactured by rolling the electrode mixture by a roll press machine (SA-602, Tester Sangyo Co., Ltd.) at 160° C. and linear pressure: 0.4 t/cm. The thickness of the electrode sheetwas 110 μm.

In comparative example 1, an electrode mixture was produced by mixing the raw materials at once. That is, as shown in, the NCM-1, the NCM-2, the PTFE, the CNT, and the PVdF were put into the above-mentioned mixer (MP5B, Nippon Coke Co., Ltd.), mixed at 300 rpm for 180 seconds, and then mixed at 5000 rpm for 500 seconds. The electrode mixture was thereby produced. The method of manufacturing the electrode sheet from the electrode mixture was the same as that in example 1.

Comparative example 2 is the same as comparative example 1 except for the former involves a kneader. That is, as shown in, the mixture mixed in the mixer in comparative example 1 was put into the above-mentioned kneader and kneaded at 100° C. and 10 rpm for 180 seconds. An electrode mixture was thereby produced. The method of manufacturing the electrode sheet from the electrode mixture was the same as that in example 1.

The electrode sheets manufactured by the methods of example 1 and comparative examples 1 to 2 were punched with a punching blade die to produce a dogbone-shaped sample pieces each having 4 mm width. The thickness of the sample pieces was approximately 5.6 mm. Measurement was made using AGS-X, 50N load cell, and 50N clip-type gripper manufactured by Shimadzu Corporation as measurement devices, with a distance between grippers of approximately 4.0 mm and an initial strain rate of 0.33/s (tensile rate 1.3 mm/s). The measurement results are shown in.

The results shown inindicate that the tensile strength of the electrode sheetof example 1 is higher than that of the electrode sheet of comparative example 1. The tensile strength of the electrode sheet in comparative example 2 is also higher than that of the electrode sheet in comparative example 1. It can be said that electrode sheets have improved tensile strength when they are made from electrode mixtures that include fibrillated PTFE.

The electrical resistances of the sample pieces (11.25 mm) of the electrode sheets made by the methods of example 1 and comparative examples 1 to 2 were measured using an electrode resistance measurement system (RM2610, Hioki E.E. Corporation). The measurement results are shown in.

The results shown inindicate that the electrical resistance of the electrode sheetin example 1 is lower than that of the electrode sheet in comparative example 2. It can be said that when PTFE is fibrillated in producing an electrode mixture, mixing NCM-1 and PTFE, as well as NCM-2, CNT, and PVdF separately is more advantageous than mixing them together at once in terms of conductivity of the electrode sheet. This would be because the CNT is suppressed from being incorporated into the PTFE and a relatively large amount of CNT extends between the NCMs, thus forming sufficient conductive pathways in the electrode sheet.